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Lamotrigine-Valproate Interaction — MRCPsych Paper B MCQ

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HardMood DisordersLamotrigine-Valproate InteractionMRCPsych Paper B

A 32-year-old woman with bipolar I disorder is to start lamotrigine for prevention of recurrent depressive episodes. She remains on valproate because no effective or tolerated alternative has been identified, following the required specialist reviews, and all applicable MHRA Pregnancy Prevention Programme requirements are being met. She takes no other medicines that affect lamotrigine glucuronidation. Which lamotrigine titration schedule is most appropriate?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: E — 25 mg on alternate days in weeks 1–2; 25 mg daily in weeks 3–4; 50 mg daily in week 5; 100 mg daily from week 6

Explanation lettering: D = shown as A · E = shown as B · A = shown as C · C = shown as D · B = shown as E

Option B is correct. Valproate inhibits lamotrigine glucuronidation, reducing its clearance and increasing its mean half-life nearly two-fold. The licensed bipolar-disorder schedule is therefore 25 mg on alternate days for weeks 1–2, 25 mg daily for weeks 3–4, 50 mg daily in week 5, and a usual target of 100 mg daily from week 6. The initial dose and escalation must not be exceeded because high starting doses, rapid escalation and concomitant valproate increase the risk of serious rash, including Stevens–Johnson syndrome and toxic epidermal necrolysis. Option A is the schedule used without valproate or glucuronidation inducers. Option C resembles the higher-dose regimen required with enzyme-inducing medicines in the absence of valproate. Options D and E escalate too rapidly and reach an inappropriately high dose.

Reference: GlaxoSmithKline UK. Lamictal Summary of Product Characteristics, sections 4.2 and 4.5, revised November 2025. https://www.medicines.org.uk/emc/medicine/4228